1. Field of the Invention
The invention relates to gas-liquid mixing operations. More particularly, it relates to enhanced gas-liquid mixing under particular variable liquid level operating conditions.
2. Description of the Prior Art
In gas-liquid mixing operations, the Advanced Gas Reactor (AGR) system employs a down-pumping impeller positioned within a hollow draft tube in a mixing vessel to create a recirculating flow pattern in a body of liquid contained in the vessel. Because of such recirculation of the liquid downward in the hollow draft tube and upward in the vessel outside said draft tube, vortices are formed in the upper inlet area of the draft tube so as to draw feed gas from an overhead gas space within the vessel into the recirculating liquid passing downward into the draft tube, as disclosed in the Litz patent, U.S. Pat. No. Re. 32,562.
Satisfactory vortex development for such gas-liquid mixing purposes depends, among various practical operating factors, on the maintaining of a proper liquid level above the top of the draft tube. If the liquid level were to rise above this operating level, the formation of vortices would be impeded, and the gas ingestion rate would decrease essentially to zero. Thus, operating at liquid levels above the optimum level can substantially reduce the gas ingestion capabilities of the AGR system. If the liquid level falls below the top of the draft tube, all pumping action ceases.
In gas-liquid mixing operations subject to non-optimum liquid levels, therefore, the effectiveness of the AGR system can be seriously impaired. In the Litz patent, U.S. Pat. No. 4,919,849, the use of hollow gas ingestion tubes connected to a hollow shaft are disclosed as a means for drawing gas into downward pumping helical impeller means positioned in the hollow draft tube and located at non-optimum liquid levels during the course of gas-liquid mixing operations subject to variable liquid level operations.
In many gas/liquid mixing applications, particularly those in the specialty chemical and pharmaceutical areas, variations in liquid level within a vessel are very common. They may be caused by variations in the batch size processed, an increase or decrease in the volume of reactants, or the addition or removal of material as the reaction proceeds. In many processes, it is desirable to be able to recirculate a gas or gases that accumulate in the vessel head space. This is particularly the case of hydrogenation and oxygenation processes. While the above-indicated Litz patent, U.S. Pat. No. 4,919,849, addresses this matter and provides for the drawing of gas from the overhead gas space in circumstances in which the vortex development of an AGR system, and thus gas ingestion, is impeded, further improvements are desired in the gas-liquid mixing art. In particular, it is desired to provide for enhanced gas-liquid mixing in reactor vessels having very large liquid variations, e.g., as much as 8 feet or more, during the course of gas-liquid mixing operations.
It is an object of the invention, therefore, to provide an improved system for gas-liquid mixing subject to variable liquid operating level conditions.
It is another object of the invention to provide a system to maximize gas-liquid mixing and reaction rates under variable liquid level conditions.
It is a further object of the invention to provide a gas-liquid mixing system for optimizing the use of hollow gas ingestion tubes for a totally submerged helical impeller/draft tube assembly.
With these and further object in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.